In the mass-production of packages of small objects, in particular pills or similar pharmaceutical products, it is standard to form a base foil with an array of upwardly open blisters, to load at least one object into each of the blisters, and to bond a cover foil to an upper surface of the base foil so that it hermetically adheres to the base foil at webs between the blisters. This is all done continuously, with the foils moving through shaping, filling, and sealing stations at a constant travel speed. The base foil is typically a vacuum-formed thermoplastic, and the cover foil is typically a much thinner foil of metal and/or plastic that must be removed for access to the product by the consumer.
As detailed in U.S. Pat. No. 5,582,665 of Eigen, the quality of the seal between the foils at the webs between the blisters is critical. This seal is typically formed by passing the two foils between two dies having confronting outer surfaces. The lower die has an array of upwardly open pockets that fit with the blisters of the base foil, and the upper die has a surface formed with regions contacting the webs between the blisters with an array of small bumps or ridges that in fact impress a pattern into the upper cover foil. In U.S. '665 the bumps are of a shape tapering, that is becoming of smaller cross-sectional area, away from the die toward the workpiece, i.e. the foils, which may or may not have been fixed together into a laminate.
During the sealing operation with the shaped die or dies, the bumps actually are pressed into and even through the respective foil, almost always on the upwardly facing cover-foil side, and in many cases also on the downwardly facing base-foil side. Deformation is plastic and permanent with an imprint depth that can even exceed a thickness of the cover foil. This imprint depth has been determined to be critical to the web seal, for instance to meet the high standards of the European Commission, the Pharmaceutical Inspection Convention (PIC), and the US FDA, among others.
The imprint depth is a function of the composition, temperature, and thickness of the foil, the shape of the tool, the force applied to the tool, and other minor factors, so the imprint depth can vary when any of these factors changes. On the other hand experience has shown that the imprint depth must be a certain size for a proper seal. Too big and the foils might be pierced or damaged, too small and the foils are not adequately bonded. Since the primary factor determining imprint depth in a system is the pressure applied through the tool to the workpiece, it is standard in the prior art to use this factor—die pressure—to calculate imprint depth. This however does not take into account the other factors affecting imprint depth, the critical factor, so it is inaccurate at best. The actual seal might be inadequate, for example, because as a result of down time a portion of the workpiece gets very hot and is therefore very soft, so the imprint with a given pressure will be too deep.